Memory system and data writing method

ABSTRACT

A data writing method is disclosed. In a memory system comprising a NAND flash memory and a controller which controls the memory, the memory system storing data provided from a host to the NAND flash memory, the data writing method comprises a steps of specifying a column address in which a column failure which has occurred in the NAND flash memory by the controller, and a step of, during writing into the NAND flash memory, writing data of a first logic level into a memory cell which corresponds to the specified column address regardless of write data provided from the controller.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority fromprior Japanese Patent Application No. 2004-225026, filed Aug. 2, 2004,the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a memory system and a data writingmethod utilizing the memory system, and more particularly, to measuresagainst a bit line failure of a nonvolatile semiconductor memory.

2. Description of the Related Art

In a nonvolatile semiconductor memory such as a NAND type flash memory,cell failure may occur before or after shipment of the memory. Suchnonvolatile semiconductor memory, which may include cell failure, may bestill utilized as storage devices by way of control of a controller. Thecontroller controls the nonvolatile semiconductor memory onblock-by-block basis, the block serving as an erasure unit when data isrewritten in the nonvolatile semiconductor memory. Refer to, forexample, Jpn. Pat. Appln. KOKAI Publication No. 7-29392. However, withthe trend of large scale integration and miniaturization of thenonvolatile flash memory, there have been substantiated not only theabove-described failure controllable in block-by-block basis but alsoanother failure called a “column failure”. The column failure givesundesirable influence on a plurality of blocks sharing the same bitline. Thus, it is impossible to manage such a column failure by way ofblock-by-block basis.

In the case where a memory cell array is managed on a block-by-blockbasis, when a write failure or an erasure failure is detected in a blockserving as a write object or erasure object, a reserved block is used inplace of the faulty block. However, when a column failure occurs, theremay exist a write failure or an erasure failure in not only a block butalso its reserved block, since these blocks generally share a bit line.When such a column failure occurs before shipment of a nonvolatilesemiconductor memory product, it is possible to take measures by using atechnique called column redundancy to program a decoder circuit so as toreplace the faulty column with a redundancy column. Refer to, forexample, Jpn. Pat. Appln. KOKAI Publication No. 2002-117692. However,when the column failure occurs after shipment of the nonvolatilesemiconductor memory, it is impossible to carry out measures by usingthe column redundancy technique. In addition, in a system including thenonvolatile semiconductor memory and controller, even when a write erroror an erasure error is detected in the nonvolatile semiconductor memoryby the controller, the controller cannot detect whether or not sucherror is caused by the column failure.

BRIEF SUMMARY OF THE INVENTION

According to an aspect of the present invention, there is provided, in amemory system comprising a NAND flash memory and a controller whichcontrols the NAND flash memory, in which data inputted from a host isstored in the NAND flash memory, a data writing method comprising:

specifying a column address of a column of the NAND flash memory, inwhich a column failure has occurred, by the controller; and

writing data of a first logic level to a memory cell which correspondsto a specified column address in writing to the NAND flash memory,regardless of data inputted from the controller.

According to another aspect of the present invention, there is provideda memory system comprising:

a memory cell array to which data is written on a page-by-page basis;

a page buffer which holds data for one page written into the memory cellarray;

an input terminal which receives write data from the outside; and

a mask circuit connected to the input terminal and the page buffer, themask circuit masking part of the write data received by the inputterminal,

wherein, in the case where an electric charge is not injected in amemory cell of the memory cell array, a latch circuit of the page buffercorresponding to the memory cell holds a first logic level, and

the mask circuit masks part of the write data by a signal of the firstlogic level.

According to still another aspect of the present invention, there isprovided a memory system comprising:

a NAND flash memory including a memory cell array and a page bufferwhich holds data to be stored in the memory cell array or data read fromthe memory cell array, wherein the memory cell array has a data storageregion in which inputted user data is stored and an expected valuestorage region in which data of a predetermined pattern is stored inadvance, and data storage region and the expected value storage regionshare a bit line, and

a controller which reads data from the expected value storage region andcompare the read data with the predetermined pattern.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1 is a block diagram depicting an example of a configuration of aNAND flash memory in accordance with a first embodiment of the presentinvention;

FIG. 2 is a block diagram of a storage system in accordance with asecond embodiment of the present invention;

FIG. 3 is a flowchart showing procedures for detecting a column failurein accordance with the second embodiment;

FIG. 4 is a circuit diagram of a NAND flash memory card in accordancewith a third embodiment of the present invention;

FIG. 5 is a block diagram of a memory card in accordance with the thirdembodiment;

FIG. 6 is a flowchart showing procedures for detecting a column failurein accordance with the third embodiment;

FIG. 7 is a block diagram of a memory card in accordance with a fourthembodiment of the present invention;

FIG. 8 is a flowchart showing procedures for detecting a column failurein accordance with the fourth embodiment;

FIG. 9 is a flowchart showing procedures for detecting a write error inaccordance with a sixth embodiment of the present invention;

FIG. 10 is a flowchart showing procedures for detecting a write error inaccordance with the sixth embodiment;

FIG. 11 is a block diagram of a NAND flash memory in accordance with aneighth embodiment of the present invention; and

FIG. 12 is an overview of a system to which the present invention isapplied.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, embodiments of the present invention will be described withreference to the accompanying drawings.

First Embodiment

FIG. 1 is a block diagram depicting an example of a configuration of aNAND flash memory in accordance with a first embodiment of the presentinvention. As shown in FIG. 1, a NAND flash memory 1 has, a memory cellarray 3, and a page buffer 5 which holds data to be stored in the memorycell array or data read from the memory cell array. The memory cellarray 3 has: a data storage region 7 which stores user data to beinputted; and an expected value storage region 11 which shares a bitline 9 with the data storage region 7, but which is not used to storethe user data. The memory cell array includes a plurality of blocks.Individual blocks include a plurality of pages. Data on predeterminedpatterns are stored in advance in the expected value storage region 11before a column failure occurs, thereby making it possible to detect acolumn failure while the data on these predetermined patterns aredefined as an expected value. In the case where the column failureexists, the data actually read from the expected value storage region 11does not match the above-described predetermined pattern stored inadvance.

In the case where an error has been detected when the user data iswritten into the data storage region 7, the error is assumed to be anerror caused by a failure of a specific memory cell included in a pagetargeted for writing, an error caused by a failure of a block, or anerror caused by a failure of a bit line which extends to a memory cellarray (column failure). In this case, data is read from the expectedvalue storage region, and if the thus read data does not match apredetermined pattern, it can be judged that a column failure hasoccurred.

The column failure includes a bit line leakage featured in that a memorycell transistor is as if it were always electrically conductive and abit line open featured in that a memory cell transistor is as if it werealways electrically nonconductive. In the case where a column failure isdue to the bit line leakage, data read from a memory cell in which thecolumn failure occurs is “1”. On the other hand, in the case where acolumn failure is due to the bit line open, data read from a memory cellin which the column failure occurs is “0”. Thus, as a predeterminedpattern of data serving as an expected value, “0000 . . . 00” (namely,“0” is set to all bits on one page) is used to detect the bit lineleakage, whereas “FFFF . . . FF” (namely, “1” is set to all bits) isused to detect the bit line open. For example, in the case where onlythe bit line leakage is likely to occur for a reason relevant to aprocess of manufacturing the nonvolatile semiconductor memory, “0000 . .. 00” may be stored as pattern data in the expected value storageregion. In addition, in the case where only the bit line open is likelyto occur, “FFFF . . . FF” may be stored as predetermined pattern data inthe expected value storage region. In the case where both of the bitline leakage state and the bit line open state may occur, for example,“0000 . . . 00” and “FFFF . . . FF” are stored as expected values in aneven numbered page and an odd numbered page, respectively.

For example, in the case where a bit line open occurs with a bit linewhich falls into the beginning of a page, the read data is actually“7FFF . . . FF”, although it should be “FFFF . . . FF”. This state isdetected in a plurality of pages of the expected value storage region11, thus making it possible to assume that a column failure hasoccurred.

In this way, in the NAND flash memory according to the presentinvention, a different data from that read when a column failure occursis stored as an expected value in an expected value storage region.Thus, when an error is detected at writing of user data, a columnfailure can be detected by reading data from the expected value storageregion having a bit line shared with a memory cell in which the error isdetected.

In order to increase the precision of column failure detection, it ispossible to assign a plurality of blocks to the expected value storageregion 11. If the data read from pages in different blocks each does notmatch a predetermined pattern stored in the expected value storageregion, it indicates that a column failure occurs instead of indicatinga failure caused by a specific block.

Second Embodiment

The present embodiment describes a method of detecting the presence orabsence of a column failure by utilizing a general read command (forexample, 00h) which a NAND flash memory comprises in the above-describedNAND flash memory having the expected value storage region.

FIG. 2 shows a storage system which includes a controller 20 and a NANDflash memory 22. The controller 20 comprises a CPU 25, a buffer RAM 26which temporarily stores data received from a host, and an errorchecking & correction (ECC) circuit 27. The controller 20 comprises theECC circuit 27, thus making it possible to specify a column address inwhich a column failure occurs within the range of the correctioncapability of the ECC circuit 27 by reading data contained in a page byutilizing a general read command (for example, 00h) which the NAND flashmemory provides after an error has been detected in an operation forwriting data into such a page contained in a block of the NAND flashmemory 22. Similarly, it is possible to specify a column address inwhich a failure occurs also in the case where an ECC error has beendetected in an operation for reading data from a page in a block. Inaddition, the column address specified by reading data contained in theexpected value storage region is compared with the column addressspecified in a data storage region by utilizing the general read commandand ECC circuit 27. When these column addresses match each other, it canbe said that a column failure occurs.

FIG. 3 is a flowchart showing procedures for detecting a column failuredescribed above. First, the controller inputs a data input command (forexample, 00h), an address, data, and a program commands (for example,10h) via an I/O terminal to the NAND flash memory, and starts datawriting into a page contained in a block of the NAND flash memory(S301). Next, the controller monitors a signal outputted to aReady/Busy# terminal of the NAND flash memory until the NAND flashmemory has outputted a Ready signal (S302). When the Ready signal isdetected, the controller issues a status read command (for example, 70h)to the NAND flash memory via the I/O terminal, and reads a write statusrelevant to the NAND flash memory from the I/O terminal of the NANDflash memory (S303). Next, the controller makes a status determination(S304). When the status is “Pass”, the controller terminates the writeprocessing (S314). When the status is “Fail”, the controller reads datafrom the page on which the write processing has been carried out (S305).Next, the controller makes a check using an ECC circuit on the readdata, and specifies a column address in which an error occurs (S306).Next, the controller stores the specified column address in an internalregister (S307). Next, the controller reads data from the expected valuestorage region of the NAND flash memory (S308). Next, the controllermakes a check using the ECC circuit on the data read from the expectedvalue storage region (S309), and determines that, if no ECC error isdetected, the write error occurs on only a specific page (S313). In thecase where the ECC error has been detected, the controller stores thecolumn address in which the error occurs in the internal register(S310). Next, the controller compares the column addresses stored in theabove steps S307 and S310 (S311), and if these addresses do not matcheach other, the controller determines that the write error occurs ononly a specific page (S313). When the column addresses match each other,the controller determines that the write error occurs due to a columnfailure (S312).

When the write error has been detected, the controller 20 can specify acolumn address in which a failure occurs without using the ECC circuit27. This is because the controller holds in its internal buffer RAM 26the data to be written into the NAND flash memory 22. The controller 20reads data by a general read operation from a region of a page address(row address) in which the write error has occurred in the NAND flashmemory after the write error has been detected. Then, the controller canspecify the column address in which the write error has occurred bycomparing the data held in the buffer RAM 26 with the data read from aregion of the page address. With this technique, even if the number ofbits with which a write error occurs exceeds the correction capabilityof the ECC circuit 27, it is possible to detect the write error. Whenthe detected column address matches a column address specified byutilizing the expected value storage region described previously, itbecomes possible to detect a column failure.

Third Embodiment

A NAND flash memory in the present embodiment itself comprises afunction for detecting a bit line leakage or a bit line open.

As shown in FIG. 4, a NAND flash memory 30 has a memory cell array 31,and a page buffer 32 which holds data to be stored in this memory cellarray or data read from the memory cell array 31. The page buffer 32includes a latch circuit 34 connected to a bit line 33. In FIG. 4,although only one bit line 33 and one latch circuit 34 are described, anactual NAND flash memory includes a plurality of bit lines and latchcircuits corresponding to a page length of the memory cell array 31. Inthe following description, the latch circuit 34 holding “1” denotes thata node 35 connected to a bit line of the latch circuit 34 indicates “1”.Data contained in the latch circuit 34 is read from an I/O terminal 37via a clocked inverter 36.

In the present embodiment, the NAND flash memory 30 is compatible with aread command for column failure detection which is different from ageneral read command (00h). When this read command for column failuredetection is entered, a command decoder and state machine 38 controlsthe memory cell array 31, and an electrical potential is applied tocontrol gates of these memory cells, such that all of the memory cellsenter a forced conductive state or a nonconductive state. Then, the dataaccording to the electrical potential of the bit line 33 is captured bythe latch circuit 34 configuring the page buffer 32 of the NAND flashmemory 30. In the case where all the blocks contained in the memoryarray 30 enter a selected state and in the case where all the memorycells enter the forced conductive state, the node 35 at the bit lineside of the latch circuit 34 configuring the page buffer 32 indicates“1” if a bit line open failure occurs with the bit line 33. In the casewhere all the memory cells enter the forced conductive state, however,the note at the bit line side of the latch circuit 34 indicates “0” ifno bit line open failure occurs. In addition, in the case where all theblocks contained in the memory array 31 enter the deselected state, thenode 35 at the bit line side of the latch circuit 34 indicates “0” if abit line leakage failure occurs with the bit line 33. In the case whereall the blocks contained in the memory array 31 enter the deselectedstate, however, the node 35 at the bit line side of the latch circuit 34indicates “1” if no bit line leakage failure occurs.

FIG. 5 is a block diagram of a memory card 40 which includes such a NANDflash memory 30 and a controller 42 which controls the memory. Referenceis made to FIG. 4 for a detailed configuration of the NAND flash memory30. In such a memory card 40, the controller 42 inputs theabove-described command for column failure detection from an I/Oterminal 43 to the NAND flash memory 30. This command is provided as acommand for bit line open detection which causes all blocks to enter aselected state and establishes all memory cells in the forced conductivestate (for example, AAh) or a command for bit line leakage detectionwhich causes all the blocks contained in the memory array to enter thedeselected state (for example, ABh). The NAND flash memory 30 enters acolumn failure detecting operation upon the receipt of this command, anda Busy state is outputted to a Ready/Busy# terminal 44. When the columnfailure detecting operation terminates in the NAND flash memory 30, aReady state is outputted to the Ready/Busy# signal terminal 44. Inresponse to this Ready state, the controller clocks an RE# signalprovided to an RE# terminal 45 and reads the contents of a page buffercontained in the NAND flash memory 30. In addition to the above,although signals such as a power supply Vdd/Vss, an address latch enableALE, a command latch enable CLE, and a chip enable CE# are provided fromthe controller 42 to the NAND flash memory 30, these signals are omittedin FIG. 5. For example, in the case where a bit line enters an openstate, an electrical potential of that bit line during reading should bealways detected as “1”, and, as a result of entering a command for bitline open detection, “0” should be detected in the data read from thepage buffer. This is because the data held in the latch circuit 34 isread after being inverted by means of the clocked inverter 36.

In general, data is written into the NAND flash memory 30 in order ofdata input command entry (for example, 80h), address input, write datainput, and write command entry (for example, 10h). The inputted data isfirst stored in the page buffer 32, and is programmed to the memory cellarray 31 in response to write command entry. A verifying operation iscarried out on a bit by bit basis during programming, and “1” is set tothe latch circuit 34 of the page buffer 32 from a column in whichprogramming has completed. Programming is carried out again for a memorycell of a column in which “0” is set to the latch circuit 34. If all thebits on the page buffer are set “1” in a predetermined write count, itimplies that a write operation has normally completed. On the otherhand, if “0” more than at least 1 bit exists, it implies that the writeoperation has normally completed. In the NAND flash memory 30, it isdetermined whether or not the write operation has normally completedaccording to whether or not all the bits of the page buffer 32 are thusset to “1”.

The controller 42 senses that a signal outputted to the Ready/Busy#terminal 44 of the NAND flash memory 30 has entered the Ready state,whereby the write operation has completed. When the controller 42 sensesthat the write operation has completed, the controller determines thatthe write operation has normally completed by using a state read command(for example, 70h). In the case where a write error has been detected atthis stage, the controller determines whether or not a bit line leakagefailure occurs by utilizing the bit line leakage detecting command.

FIG. 6 is a flowchart illustrating this method. Like processes in FIG. 6are designated by like reference numerals, and a duplicate descriptionis omitted here. When “Fail” is detected in status determination afterwriting into a page, the controller issues a bit line leakage detectingcommand to the NAND flash memory (S601). Next, the controller reads abit line leakage detection result from the NAND flash memory (S602).When the controller detects a bit line leakage, the page write error isdetermined to be caused by a bit line leakage (S604). In the case whereno bit line leakage failure is detected, the controller determines thatthe write error is specific to the page (S605). A process for detectingthe column failure may be executed after an error has been detectedduring data read or erasure as well as immediately after the write errorhas occurred. Alternatively, this detecting process may be executedimmediately after starting up a memory card or a host system orimmediately before terminating it.

An example of a configuration of the memory cell array 31 in the NANDflash memory 30 will be described in detail. The memory cell array 31is, in its page length, any of 528 bytes (data portion 512bytes+redundancy portion 16 bytes); 2112 bytes (data portion 2048bytes+redundancy portion 64 bytes); and 4224 bytes (data portion 4096bytes+redundancy portion 128 bytes). The redundancy portion used here isdifferent from a redundancy portion composed of a reserved bit line foruse in column redundancy before shipment. In the case where an 8-bitarray (so called ×8) configuration is provided, a page length per I/O is528 bits, 2112 bits, or 4224 bits. The memory cell array 31 includes aplurality of blocks, and one block becomes a unit during erasure. Oneblock includes, for example, 64 pages. It is possible to associate awrite/read circuit for the memory cell 31 with two values or fourvalues. An example of a configuration of the memory cell array describedhere can be applied to a NAND flash memory according to anotherembodiment without being limited to the present embodiment.

Fourth Embodiment

Referring to FIG. 7, a description will be given with respect to atechnique for specifying a column address in which a bit line leakagefailure occurs in a NAND flash memory capable of reading contents of apage buffer as they are without resetting them after a write operationhas completed.

A controller 50 comprises a CPU 51, a buffer RAM 52, a faulty addressregister 53, and an ECC circuit 54. The controller is connected to aNAND flash memory via a signal line such as I/O, Vdd, Vss, Ready/Busy#,command latch enable CLE, address latch enable ALE, chip enable CE#,read enable RE#, and write enable WE#.

In order to detect a bit line leakage, the controller 50 first makes anoperation for writing data on all “0”s into a page of a block. Then,after the write operation has been completed, the contents of the pagebuffer are read as they are without resetting them. For this purpose,the controller inputs a register read command (for example, E0h) to theNAND flash memory via the signal line I/O. After this command has beencaptured by the NAND flash memory in response to the write enable WE#signal, the address latch enable ALE signal is established in adeselected state and the RE# signal is clocked, thereby making itpossible to read the data contained in the page buffer without resettingthe contents of the page buffer.

If the write operation has normally completed, all the data contained inthe page buffer should be “1”. In the case where the write operation hasnot normally completed, however, part of the data contained in the pagebuffer is set to “0”. By detecting the position of the “0” data, thecontroller can detect a column in which a write error has occurred. Byonly this operation, it is impossible to determine whether a failure ina specific cell has occurred or a failure due to a bit line leakage hasoccurred. The controller stores a faulty column address in the faultyaddress register 53 incorporated in the controller. The controller againmakes an operation for writing data on all “0”s in to a page of a blockwhich is different from the previous block. Then, the controller readsthe contents of the page buffer without resetting them after the writeoperation has completed. If the write operation has normally completedand all the data contained in the page buffer are set to “1”, it can bejudged that the write error has not occurred due to a column failure. Ifthe write operation has not normally completed and part of the datacontained in the page buffer is set to “0”, a column address in afailure occurs is specified, and the thus specified column address iscompared with a column address stored in the faulty column addressregister 53. If these addresses match each other, it can be determinedthat a write error which occurs is caused by a bit line leakage.

Alternatively, it is possible to determine a bit line leakage failure bycomparison with the faulty column address specified by data read fromthe expected value storage region described previously. This method willbe described with reference to the flowchart of FIG. 8. Like processesin FIG. 3 are designated by like reference numerals, and a duplicatedescription will be described here. If Fail is detected by statusdetermination after writing into a page, the controller reads datacontained in the page buffer as is by using a resistor read (E0h)command (S805). Next, the controller specifies a bit set to “0” fromamong the data read from the page buffer, and specifies a column addressin which an error occurs (S806). Next, the controller stores this columnaddress in the faulty column address register (S807). Reference is madeto a description of FIG. 3 for the remaining processes.

Fifth Embodiment

As described previously, in the case where a controller comprises an ECCcircuit, the controller reads the data of the page by utilizing ageneral read command (for example, 00h) after a failure has beendetected in an operation for writing into a page of a block, therebymaking it possible to specify a column address in which a failure occursin the range of the correction capability of the ECC. Similarly, in anoperation for reading a page in a block, it is also possible to specifya faulty column address in the case where an ECC error has been detectedin the ECC circuit of the controller. However, by only this operation,it is impossible to specify whether a failure is limited to only a pagetargeted for writing, whether a failure occurs in the same bit line, butis limited to the inside of the block, or a column failure occurs. Incontrast, a finding a plurality of column addresses in which a failureoccurs are stored, thereby making it possible to estimate that a failurecaused by a bit line occurs if a finding that a failure occurs in acommon column address is obtained. It is desirable that these items ofstorage information are information relating to a page or a blockaddress in a wide range connected to the same bit line. However, if afailure occurs at the same column position on each of the pages in atleast two or more different blocks, it may be judged that a columnfailure occurs. A timing of storing these faulty column addresses may bewhen a faulty column address has been obtained by reading data on thepage immediately after a write error has occurred on a page indicated byan address, or alternatively, when faulty column address information isobtained, the information being detected by the ECC circuit duringgeneral read operation. In addition, in the case where an erasurefailure has been detected for a block, it is possible to store faultycolumn addresses similarly if a page in the block is read. With respectto storage of these items of storage information, there are proposed: amethod of storing information in a RAM of a controller or a faultyaddress register and making a determination from the information storedonly during system operation (during power supply); and a method ofstoring information in a nonvolatile manner in a region of the NAND-typeflash memory in which no failure occurs, and making determination fromthe information obtained in a system storage operation state.

When a write error has been detected, the controller can specify acolumn address in which a failure occurs without using an ECC circuit.This is because the controller holds data to be written into the NANDflash memory in its internal buffer RAM. The controller reads data by ageneral read operation from a region of a page address (row address)contained in a block in which the write error has occurred after writeerror detection. Then, when the data held in the buffer RAM is comparedwith the data read from a region of the page address, the controller canspecify a column address in which a write error has occurred. Thiscolumn address is stored in a faulty address register. Next, theoperation similar to the above is made for a page of a block which isdifferent from the above. In the case where the obtained two columnaddresses match each other, it can be judged that a column failureoccurs. With this technique, it is possible to carry out detection evenin the case where the number of bits with which a write error occursexceeds the correction capability of the ECC circuit.

Sixth Embodiment

As described previously, all the data contained in the page buffer inthe NAND flash memory has been set to “1” during writing, whereby it isjudged that the writing operation has normally completed. When,programming, namely, an electric charge is implanted into the memorycell, “0” data is held in a latch circuit which configures the pagebuffer. Then, a high voltage of about 20 V is applied to a word line(control gate) of a page targeted for the writing operation, whereby anelectric charge is implanted into a floating gate of a memory cell inaccordance with a tunneling phenomenon. In addition, during a verifyingoperation, data is read from the memory cell to a latch, whereby, if anelectric charge is sufficiently implanted into the floating gate, noelectric conductivity is obtained even if a read voltage is applied to amemory cell transistor, and thus, a bit line indicates “1”. The bit lineelectrical potential forcibly inverts a latch circuit, and “1” data isheld in the latch circuit. Such a type of circuit is described in FIG. 4explained previously, and is also disclosed in U.S. Pat. No. 5,726,882.In the case where a bit line leakage failure occurs, however, even if anelectric charge is sufficiently implanted into the floating gate, anelectrical potential of a bit line is kept to be “0”, thus the electriccharge implantation into the floating gate is as if it were incomplete.Thus, in the NAND flash memory, programming and verifying operations arerepeated up to the predetermined maximum count.

When the writing operation is started in the NAND flash memory, a signalof a Ready/Busy# terminal which the NAND flash memory has is set to Busy“0” such that a controller connected to the NAND flash memory can detectthis state, and the signal is set to Ready (“1”) at the same time aswhen the writing operation completes. In addition, in the NAND flashmemory, information indicating whether or not the writing operation hasnormally terminated is written into a Pass/Fail bit of an internalstatus register during a Busy period. Thereafter, the controller inputsa status read command (for example, 70h) to an I/O terminal of the NANDflash memory, and determines whether or not the writing operation hasbeen normally completed by the Pass/Fail bit on the status register, thebit being outputted from the NAND flash memory to the I/O terminal. Inthe case where a bit line leakage occurs, it is possible to finallydetect a write error by the status read command. However, theprogramming count in the NAND flash memory has reached a maximum. Thus,a write speed of the NAND flash memory is lowered, so that theperformance of a card or a storage system using the NAND flash memory islowered. For example, Toshiba 512M bit NAND flash memory TC58512FTpresumes that a typical write time tPROG is 200 microseconds. In thecase where the programming count has reached the maximum, however, tPROGmay be 1000 microseconds.

A bit line failure itself in the NAND flash memory does not occurfrequently, thus, making it possible to correct data after an error hasoccurred by an ECC circuit at the controller side. Therefore, in somesystems, it becomes possible to reduce the write error count which theNAND flash memory notifies the controller by providing the NAND flashmemory with a pseudo-pass function. The pseudo-pass function used heredenotes that, in the case where a programming failure of data on a pagehas occurred in the NAND flash memory, and if the program failure iswithin a predetermined allowable bit number, it is considered thatprogramming has normally terminated for all the bits, and the NAND flashmemory returns Pass in a pseudo manner to the status read commandimmediately after the writing operation has completed. Here, it ispossible to set the number of faulty bits to be ignored from thecontroller side to the NAND flash memory. The controller can reduce thefrequency of error handling after writing by setting the number of bitsto at least one bit or more in accordance with the correctionperformance of its own ECC circuit. Namely, although an error occurs inthe NAND flash memory, the error is within the range of the correctioncapability of the ECC circuit. Thus, the NAND flash memory does notnotify the controller side of the occurrence of the error, therebymaking it unnecessary for the controller to execute error handling.After a column failure has been detected by the technique describedpreviously, the controller may change the number of allowable bits in apseudo-pass operation of the NAND flash memory. For example, theallowable bit number of the pseudo-pass is set to 0 (namely, a state inwhich no pseudo-pass is allowed). Then, when a bit line leakage failurehas been detected with respect to a column, the controller changes to 1the allowable bit number of the pseudo-pass in the NAND flash memory.This change is made by inputting a pseudo-pass setting command to theNAND flash memory. After the allowable bit number of the pseudo-pass hasbeen changed to 1, the NAND flash memory determines that the writingoperation normally terminates even if a bit line leakage failure occursin one column. Thus, when the controller has read a status register, thewrite result is obtained as “Pass”. A case in which the allowable bitnumber of the pseudo-pass is changed from 0 to 1 has been describedhere. This allowable bit number can be changed according to the capacityof the controller's ECC circuit and a state (frequency) of theoccurrence of a failure in the flash memory.

A writing operation and a pseudo-pass operation in the NAND flash memorywill be described along the flowchart of FIG. 9. First, in step S61, awriting start operation is started, and a program count counter N is setto 1. Next, in step S62, a programming operation for transferring datafrom a page buffer to a memory cell array is carried out. Here, aprogram voltage Vpp of about 20 V is applied to a word line of anactually selected page. Next, in step S63, verify and read operationsare carried out. Here, in a latch from among latches configuring thepage buffer, the latch corresponding to the memory cell in which anelectric charge has been sufficiently implanted, data is inverted from“0” to “1”. Next, in step S64, it is determined whether or not all thedata contained in the page buffer are set to “1”. When the determinationresult is affirmative, processing goes to step S65. When thedetermination result is negative, processing goes to step S66. In stepS66, the programming count N is incremented by 1. Next, in step S67, itis determined that the programming count N has reached the set maximumvalue NMAX. If the determination result is negative, processing revertsto step S62. When the determination result is affirmative, processinggoes to step S68. In step S68, it is determined whether or not thenumber of bits set to “0” in the page buffer is equal to or smaller thana set allowable value. When the determination result is affirmative,processing goes to step S65. When the determination result is negative,processing goes to step S69. In step S65, “Pass” is set in the statusregister, and in step S69, “Fail” is set in the status register. Next,in step S70, a Ready/Busy# signal is returned to a Ready status, and thewrite operation completes.

In this example, after the programming count has reached the maximum inthe NAND flash memory, it is determined whether or not the number o bitsset to “0” in the page buffer is equal to or smaller than an allowablenumber of bits. However, if the NAND flash memory is a circuit capableof processing determination of step S68 at a high speed, it is alsopossible to employ a flow of processing which is different from theabove. This flow of processing will be described along the flowchart ofFIG. 10.

First, in step S71, a write start operation is carried out, and aprogram count counter N is set to 1. Next, in step S72, a programmingoperation for transferring data from a page buffer to a memory cellarray is carried out. Here, a program voltage Vpp of about 20 V isapplied to a word line of an actually selected page. Next, in step S73,verifying and reading operations are carried out. Here, in a latch fromamong latches configuring the page buffer, the latch corresponding tothe memory cell in which an electric charge has been sufficientlyimplanted, data is inverted from “0” to “1”. Next, in step S74, it isdetermined whether or not all the data contained in the page buffer areset to “1”. When the determination result is affirmative, processinggoes to step S75. When the determination result is negative, processinggoes to step S76. In step S76, it is determined whether or not thenumber of bits set to “0” in the page buffer is equal to or smaller thana set allowable value. When the determination result is affirmative,processing goes to step S75. When the determination result is negative,processing goes to step S77. In step S77, the programming count isincremented by 1. Next, in step S78, it is determined whether or not theprogramming count N has reached the set maximum value NMAX. When thedetermination result is negative, processing reverts to step S72. Whenthe determination result is affirmative, processing goes to step S79. Instep S75, “Pass” is set in the status resistor, and in step S79, “Fail”is set in the status register. Next, in step S70, a Ready/Busy# signalis returned to a Ready state, and the write operation completes. In thisexample, before the programming count in the NAND flash memory hasreached the maximum, it is properly determined whether or not the numberof bits set to “0” in the page buffer is equal to or smaller than anallowable number of bits. Therefore, if the number of bit lines in whicha bit line leakage failure occurs is equal to or smaller than theallowable value, the programming count does not reach the maximum value.

Seventh Embodiment

The present embodiment describes write error detection in the case wherea NAND flash memory in which a column failure has detected is providedwith a pseudo-pass function.

While the sixth embodiment describes utilizing the pseudo-pass functionof the NAND flash memory, a controller can obtain a pseudo-Pass resultafter an internal time-out has occurred in a write operation of the NANDflash memory (after the programming count has reached the maximum).Thus, for example, if a bit line leakage failure occurs, the maximumprogramming count has been reached every time the writing operation iscarried out, and thus, the writing performance of the entire systemutilizing the NAND flash memory is lowered. Accordingly, the presentembodiment provides a method of, even in the case where a bit lineleakage failure has occurred, obtaining a write Pass result withoutcausing the lowered system performance.

Although described previously, in the NAND flash memory, during datawriting, the data inputted from the controller is temporarily stored inthe page buffer in the flash memory, and then, the writing operation isexecuted for the cell array which falls under the selected address atthe same time. If an attempt is made to program “0” data in a memorycell of a column in which a bit line leakage failure occurs, a latchcircuit is not forcibly inverted as described previously, and theprogramming operation is repeated up to the maximum count. Then, in thecase where the controller writes data into the NAND flash memory, “1” isalways written into the column in which a bit line leakage failuredetected in advance occurs, regardless of a value of data which a hostdevice attempts to essentially write. Namely, the essential data ismasked by “1”. In this case, data falling under a faulty address ismasked by “1” regardless of a value of write data. Accordingly, it ispresumed that correction is carried out by an ECC circuit of thecontroller during read. In the case where masking of a faulty address iscarried out, there is proposed a method of carrying out masking by a bitunit “1”, namely, carrying out masking by byte (or word) units “FFh” (orFFFFh”). It is possible to freely select any method depending on a unitof address information obtained by a system and a method for detectingthe column failure. Further, when masking is carried out, there isproposed a case where the system inputs the data masked in advance inthe NAND flash memory, and a method of transferring data to the flashmemory, and then, masking data relating to the address on an internalpage buffer of the NAND flash memory.

In addition, in the case where the NAND flash memory has a function forremoving a specific bit line from a verification target during writingand during erasure, or alternatively, a function for always writing “1”or “FFh” into a memory cell of a specific column address in response todata writing, of course, the system utilizes this function, so that asimilar object can be achieved.

Eighth Embodiment

The present embodiment describes a NAND flash memory having a functionfor always writing “1” or “FFh” in a memory cell of a specific columnaddress.

As shown in FIG. 11, a memory cell array of the NAND flash memory has aROM fuse region 81 and a user region 82. The ROM fuse region 81 is partof a memory cell array, but is a region used to store a variety of datafor controlling the NAND flash memory itself. A page buffer A83 isconnected to the memory cell array via a bit line so as to write datafrom the page buffer A83 into a masking register 85 via a clockedinverter 84. The masking register 85 is designed to write data back tothe page buffer A83 via one input of an OR circuit 86. The other inputof the OR circuit 86 is connected to a page buffer B87. The page bufferB87 is connected to the page buffer A83 via a MOS transistor 88. Whendata is read from the memory cell array to the outside, data istransferred from the page buffer A83 to the page buffer B87 via the MOStransistor 88. The page buffer A83, the masking register 85, and thepage buffer B87 each has a bit length which is equal to that of a pagelength of the memory cell array. The OR circuit 84 and the MOStransistor 86 are provided to be associated with this bit width. In theNAND flash memory, all the data set to “1” is described on apredetermined page of the ROM fuse region 81 in a state in which no bitline leakage failure occurs. When the NAND flash memory senses thatpower is supplied, a state machine in the NAND flash memory reads datafrom the predetermined page in the ROM fuse region 81, and stores thedata obtained by inverting the data stored on the predetermined page,namely, all “0” in the masking register 85 via the page buffer A83 andthe clocked inverter 84. With respect to data provided from the outsideof the NAND flash memory to an I/O terminal 89, the data being capturedin the page buffer B87, all the data contained in the masking registerare set to “0” at this time, and therefore, is stored in the page bufferA83 as is. If a bit line leakage occurs in a column at one time point,with respect to the data stored in the masking register immediatelyafter the NAND flash memory has been powered ON, the bit correspondingto the column is set 1, and the remaining data are set to “0”. Then,during subsequent writing operation, the data stored from the outside inthe page buffer B is masked in a data pattern of the masking register85, and is stored in the page buffer A83. Namely, regardless of the datainputted from the outside to the I/O terminal 89, “1” data is alwayscaptured in the page buffer with respect to a column in which a bit lineleakage has occurred, thus making it possible to avoid the programmingcount during writing operation reaching the maximum.

Further, when an attempt is made to improve reliability of the abovecircuit, all pages in the ROM fuse region may be selected by utilizingthe bit line leakage detecting command described previously instead ofreading only a page when power is supplied. In addition, a bit lineleakage detecting command is executed for at least two different blocks,whereby a data pattern may be registered in the masking register bydetecting that these two blocks match each other.

The eighth embodiment of the present invention has described a case inwhich the NAND flash memory and controller are included in the samememory card, as shown in FIGS. 2 and 5. However, the present inventionmay include a controller 130 at the side of a host 120 such as a digitalcamera without including a controller in a memory card 110 whichincludes a NAND flash memory 100, as shown in FIG. 12.

In addition, in the case where a column failure is detected when pluralitems of error information in a plurality of blocks match each other andin the case where a memory cell array of a NAND flash memory consists ofa plurality of planes, a bit line is not shared in different planes.Thus, a plurality of blocks must be selected in the same plane. If afaulty address register is provided, it is necessary to provide theregister corresponding to each plane.

Further, the NAND flash memory in accordance with the embodiments has awire width of for example, smaller than 0.1 microns, making it possibleto ensure that its capacity is equal to or greater than 2 gigabits. Inthe case of such a NAND flash memory, it is possible to use a wirematerial which includes Cu (Copper), for example.

Further, in the above-described embodiments, there has been describedthe column failure detection by using a bit line leakage detectingcommand to detect a bit line leakage. Column failure includes the bitline leakage and bit line open. Although the bit line open detection isnot described in the embodiments, the bit line open is detected by usinga bit line open detecting command. Thus, the column failure detectingcommand is defined to mean both the bit line leakage and the bit lineopen detecting command.

In the foregoing, a method of detecting a column failure and a method ofavoiding a write error which occurs due to such a column failure hasbeen described by way of a NAND flash memory. However, this columnfailure is not limited to the NAND flash memory. The present inventioncan be applied to a nonvolatile semiconductor memory system usinganother nonvolatile memory.

In addition, the present invention is not limited to the embodiments.Various modifications can occur without deviating from the spirit of theinvention at a stage at which the invention is carried out. Further,each of the embodiments includes inventions at various stages, and avariety of inventions can be excerpted in accordance with propercombinations in a plurality of constituent elements disclosed. Forexample, even if some of all the constituent elements presented in theembodiments are deleted, the constituent elements after deleted can beexcerpted as inventions in the case where (at least one of) the programsdescribed in the Background of the Invention section can be solved andin the case where (at least one of) the advantageous effects describedin the Detailed Description of the Invention section can be attained.

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details and representative embodiments shownand described herein. Accordingly, various modifications may be madewithout departing from the spirit or scope of the general inventiveconcept as defined by the appended claims and their equivalents.

1. In a memory system comprising a NAND flash memory and a controllerwhich controls the NAND flash memory, in which data inputted from a hostis stored in the NAND flash memory, a data writing method comprising:specifying a column address of a column of the NAND flash memory, inwhich a column failure has occurred, by the controller; and writing dataof a first logic level to a memory cell which corresponds to thespecified column address in writing to the NAND flash memory, regardlessof data inputted from the controller.
 2. A data writing method accordingto claim 1, wherein the step of specifying the column address includescarrying out error check and correction on data read from the NAND flashmemory, carried out after issuing data read command to the NAND flashmemory.
 3. A data writing method according to claim 1, wherein the stepof specifying the column address includes detecting a second logic levelfrom a verification result read from a page buffer of the NAND flashmemory, carried out after issuing a register read command to the NANDflash memory.
 4. A data writing method according to claim 1, wherein thestep of specifying the column address includes detecting the first logiclevel or a second logic level from data read from a page buffer of theNAND flash memory, carried out after issuing a column failure detectingcommand to the NAND flash memory.
 5. A memory system comprising: amemory cell array to which data is written on a page-by-page basis; apage buffer which holds data for one page written into the memory cellarray; an input terminal which receives write data from the outside; anda mask circuit connected to the input terminal and the page buffer, themask circuit masking part of the write data received by the inputterminal, wherein, in the case where an electric charge is not injectedin a memory cell of the memory cell array, a latch circuit of the pagebuffer corresponding to the memory cell holds a first logic level, andthe mask circuit masks part of the write data by a signal of the firstlogic level.
 6. A memory system according to claim 5, wherein the maskcircuit has a bit width which is equal to that of the page buffer, andthe signal of the first logic level is held in a bit corresponding to acolumn address in which a column failure has been detected.
 7. A memorysystem comprising: a NAND flash memory including a memory cell array anda page buffer which holds data to be stored in the memory cell array ordata read from the memory cell array, wherein the memory cell array hasa data storage region in which inputted user data is stored and anexpected value storage region in which data of a predetermined patternis stored in advance, and data storage region and the expected valuestorage region share a bit line, and a controller which reads data fromthe expected value storage region and compare the read data with thepredetermined pattern.
 8. A memory system according to claim 7, thecontroller includes an ECC circuit which detects an error when the userdata is read from the data storage region, and when the error isdetected by the ECC circuit, reads data from the expected value storageregion and compare the read data with the predetermined pattern.